The invention relates to a process for the preparation of an impact vinylaromatic composition, i.e. a composition comprising a vinylaromatic polymer matrix surrounding rubber nodules.
Patent Application EP 0,726,280 teaches that it is possible to produce an impact polystyrene by polymerizing a vinylaromatic monomer in the presence of a polybutadiene and of a stable free radical.
Patent EP 0,048,389 teaches that it is possible to obtain an impact polystyrene in which the nodules are essentially in the form of capsules, provided that a styrene-butadiene copolymer is used as the rubber.
The process according to the invention involves a step of polymerizing at least one vinylaromatic monomer in the presence of rubber, of a polymerization initiator and of a stable free radical, the said step being such that
if (SFR) represents the number of moles of stable free radical in the polymerization mixture,
if FSFR represents the functionality of the stable free radical, i.e. the number of sites on the same molecule of stable free radical having the stable free radical state,
if (INIT) represents the number of moles of polymerization initiator in the polymerization mixture, and
if FINIT represents the functionality of the initiator, i.e. the number of sites having the free radical state that each molecule of initiator is capable of generating, then:       0.05     less than                             F          SFR                xc3x97                  (          SFR          )                                      F          INIT                xc3x97                  (          INIT          )                       less than     1    ,
and preferably:   0.05   less than                     F        SFR            xc3x97              (        SFR        )                            F        INIT            xc3x97              (        INIT        )               less than       0.5    .  
As an example of a stable free radical having a functionality FSFR of 1, mention may be made of a molecule represented by: 
in which the groups R1, R2, R3, R4, R5, R6, R7 and R8 represent alkyl radicals.
As an example of a stable free radical having a functionality FSFR of 2, mention may be made of a molecule represented by: 
in which the groups R1, R2, R3, R4, R5, R6, R7 and R8 represent alkyl radicals and n represents a non-zero integer.
As an example of an initiator having a functionality FINIT of 2, mention may be made of dicumyl peroxide.
As an example of an initiator having a functionality FINIT of 4, mention may be made of ethyl 3,3-di(tert-amylperoxy)butyrate which may be represented by: 
as it contains two xe2x80x94Oxe2x80x94Oxe2x80x94links each capable of generating two sites having the free radical state, namely xe2x80x94O+.
Using one particular [FSFRxc3x97(SFR)]/[FINITxc3x97(INIT)] ratio, a polymerization range is defined within which it is possible to control and choose the morphology and the distribution of the sizes of nodules in the impact composition.
In addition, the process according to the invention is rapid, despite the use of a stable free radical which generally tends to slow down the polymerization.
The following morphologies of rubber nodules may be obtained:
the xe2x80x9csalamixe2x80x9d-type morphology, which means that the rubber particle contains several generally approximately spherical, but non-concentric, vinylaromatic polymer occlusions;
the xe2x80x9clabyrinthxe2x80x9d-type morphology, which means that the rubber particle contains several elongate, curved, generally non-spherical and generally unsymmetrical vinylaromatic polymer occlusions;
the xe2x80x9conionxe2x80x9d-type morphology, which means that the rubber particle is approximately spherical and contains, concentrically with respect to itself, several vinylaromatic polymer occlusions contained one within another; and
the xe2x80x9ccapsulexe2x80x9d-type morphology, which means that the generally approximately spherical rubber particle contains a single vinylaromatic polymer occlusion.
The xe2x80x9csalamixe2x80x9d, xe2x80x9clabyrinthxe2x80x9d and xe2x80x9conionxe2x80x9d morphologies may be called xe2x80x9cmulti-occlusionxe2x80x9d morphologies. In general, they are substantially coarser than the capsules.
These morphologies in particular impinge on the impact properties and the gloss of the final compositions.
When, for a given rubber and a given rubber content, the grafting is less, the nodules are essentially in salami form.
Within the above-defined polymerization range, it is possible, for a given rubber and a given rubber content, to increase the proportion of nodules in labyrinth form, in onion form or in capsule form by increasing the grafting of the rubber.
It is possible to further increase the proportion of nodules in capsule form by further increasing the grafting of the rubber. It is possible to promote the formation of capsules compared with the other morphologies by using a rubber of lower viscosity.
The advantages of the invention, in particular the possibility of obtaining a high proportion of nodules in labyrinth, onion or capsule form, or even a morphology essentially in capsule form, may be obtained from a rubber not comprising a polymerized vinylaromatic monomer unit and may be obtained by using a homopolybutadiene as the rubber.
The expression xe2x80x9cgrafting of the rubberxe2x80x9d should be understood to mean the grafting of the rubber by the vinylaromatic monomer present in the polymerization mixture.
The grafting of the rubber may be increased by:
using an initiator with a higher grafting power,
increasing the polymerization temperature,
increasing the amount of initiator.
The grafting power of the initiator may be determined by routine comparative tests by analysing the morphology induced by the nature of the initiator. The grafting power of the initiator is higher the more it tends to generate the following morphologies, given in the direction of increasing grafting power: salami, then onion or labyrinth, then capsule. Very large amounts of initiator, for example so that the ratio of the product (INIT)xc3x97FINIT to the molar amount of vinylaromatic monomer is greater than 2xc3x9710xe2x88x924, or even greater than 4xc3x9710xe2x88x924 or even greater than 6xc3x9710xe2x88x924, may be introduced. Such amounts are not usual in vinylaromatic monomer polymerization processes since they cause substantial heat generation,the heat being difficult to remove and possibly dangerous. In the context of the present invention, the Applicant has found that, on account of the amounts of initiator used, less heat was generated than normally observed. This phenomenon allows the use of large amounts of initiator, thereby allowing the grafting and the particle morphology to be controlled as desired.